1. Field of the Invention
The present invention relates to a magnetic field detecting element and a method for manufacturing the magnetic field detecting element, and particularly relates to the structure of a TMR element.
2. Description of the Related Art
Conventionally, a GMR (Giant Magneto-resistive) element using a spin valve (SV) film has been widely used as a magnetic field detecting element for a hard disk drive. In recent years, a TMR (Tunnel Magneto-resistance) element has been drawing attention as a magnetic field detecting element having a higher sensitivity. The TMR element is an element that is formed by stacking a lower layer made of magnetic material, nonmagnetic and nonconductive tunnel barrier layer, and an upper layer made of magnetic material in this order. In one example, the magnetization direction of the lower layer is fixed relative to an external magnetic field (this layer may be called a pinned layer), and the magnetization direction of the upper layer can be changed in accordance with the external magnetic field (this layer may be called a free layer). When a sense current is applied in the direction of the stacking of the element, electrons flow from the upper layer to the lower layer (or vice versa) passing through the energy barrier of the nonmagnetic and nonconductive tunnel barrier. This is called the tunneling effect. It is known that electric resistance relative to the sense current changes in accordance with the relative angle between the magnetization direction of the upper layer and the magnetization direction of the lower layer. The change in the resistance relative to the sense current (the change in magnetic resistance) is detected based on the change in the magnetization direction of the upper layer that is caused in accordance with the external magnetic field.
The TMR element detects the magnitude of an external magnetic field, and reads magnetic data in a recording medium in this way. The reproduction output of the magnetic field detecting element depends on the magneto-resistance ratio. The TMR element, which exhibits a particularly larger magneto-resistance ratio than a conventional GMR element, is suitable for providing a magnetic field detecting element having a high output. Incidentally, a film structure in which the lower layer is the pinned layer, as mentioned above, is called the bottom type. However, a film structure in which the lower layer is the free layer (the top type) is also used.
The tunnel barrier layer, which is made of nonmagnetic and nonconductive material, such as alumina, has a small thickness, which is typically about 2 nm, in order to enhance the tunneling effect. Therefore, if the tunnel barrier layer is not formed to be flat, then the film thickness varies depending on locations, and in some cases, the tunnel barrier layer is not formed at some portions. In a portion in which the film is not formed, the upper layer and the lower layer come into contact, leading to a leakage of current. If leakage of current occurs, the amount of current that can be changed due to the magneto-resistive effect is reduced, resulting in a decrease in the magneto-resistance ratio. Even if the upper layer and the lower layer are not in contact, portions having a small film thickness are apt to be damaged and to adversely influence the characteristic for withstanding voltage. From the foregoing, flatness of the tunnel barrier layer is particularly important in order to obtain a stable change in magnetic resistance.
For the reasons mentioned above, a technique for forming the tunnel barrier layer to be flat has been studied. Japanese Patent Laid-Open Publication No. 2000-101164 discloses a technique that uses amorphous magnetic material for at least a part of the lower layer in a top-type film structure. The amorphous magnetic material does not have a crystalline structure. Therefore, a layer that is formed has a surface having a small roughness, and the tunnel barrier layer, which is formed thereon, is easily formed to be flat. The above publication discloses an example of the lower layer that is formed by stacking 84Co9Fe7B (film thickness of 5 nm) and 90Co10Fe (film thickness of 2 nm) in this order (see Paragraph 103). An example is disclosed for a bottom-type film structure in which the lower layer has a stacked structure having a layer that is mainly made of Co, Ni, Fe, and a CoFeB layer. Refer to Japanese Patent Laid-Open Publication No. 2000-76623. Incidentally, the numeral on the left side of an element in the composition formula represents the atomic percent of the element in this specification.
CoFeB is suitable for forming a flat surface because CoFeB can be formed in an amorphous state by adjusting the chemical composition. Therefore, the tunnel barrier layer can be easily formed to be flat by directly forming the tunnel barrier layer on a CoFeB layer. However, the inventors found that directly forming, for example, an alumina layer, which is a typical tunnel barrier layer, on a CoFeB layer in the TMR element may lead to a decrease in the magneto-resistance ratio. In order to improve the magneto-resistance ratio, it is preferable to form a layer that is mainly made of Co, Fe, Ni, such as a CoFe layer, on a CoFeB layer, and then to form the tunnel barrier layer on the layer. However, the layer that is mainly made of Co, Fe, Ni is made of crystalline material. Therefore, even if a surface of a CoFeB layer is formed so that it is flat, it is difficult to form the tunnel barrier layer to be flat, because the layer that is mainly made of Co, Fe, Ni and that has a large roughness is disposed therebetween. If an amorphous layer is used for a part of the lower layer, then it may be easier to form the tunnel barrier layer to be flat. However, ideally, it is desirable that the tunnel barrier layer is formed directly on a flat layer.